In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node at the network side and at the terminal devices at the user side might be required to reach a sufficient link budget.
In general terms, the use of beamforming could imply that the terminal devices will be not only operatively connected to the network node via a beam but also performs a handover between (narrow) beams instead of between network nodes of different cells, or between transmission and reception points (TRPs) of one and the same network node. At higher frequency bands high-gain beamforming with narrow beams could be used due to more challenging radio propagation properties than at lower frequency bands. Each beam will only be optimal within a small area and the link budget outside the optimal beam will deteriorate quickly. Hence, frequent and fast beam switching is needed to maintain high performance. This is hereinafter referred to as beam management. One purpose of so-called beam management is thus for the network node to keep track of its served terminal devices with narrow beams (as used at the TRP of the network node and/or at the terminal devices) in order to increase coverage and throughput.
It is expected that at least some terminal devices might be provided with two or more antenna arrays, or panels, preferably pointing in different directions, in order to improve the coverage, mitigate the risk of blockage, and increase the order of spatial multiplexing. The antenna elements of the antenna arrays can be either dual-polarized on single-polarized. For example, two one-dimensional antenna arrays might be located in different directions at a terminal device.
In case both the TRP and the terminal device have (at least) four baseband chains (for example, two dual-polarized antenna arrays each), up to four-layer transmission will be possible between them. However, it could be that the radio propagation channel between the TRP and the terminal device has a channel rank that supports less than four-layer transmission. It would therefore be unnecessary, or a waste of resources, to use four antenna ports transmission for the beam management in this case.
Hence, there is a need for mechanisms for efficient determination of how many layers to use for the transmission during beam management in the cell.